Production processes in the iron and steel industry involve the formation of large amounts of by-products that create waste streams governed by costly legal disposal requirements designed to prevent environmental pollution.
In an integrated iron and steel plant, one of the most important thermal operations is the sintering of raw iron ore. In a conventional sintering process, waste products from the iron making process (a mixture of iron bearing residues, such as blast furnace dust and filter cake, commonly referred to as “reverts”), are mixed with iron ores, coke breeze, and limestone to create a sinter blend composition. The sinter blend composition is then heated at high temperatures until it is sintered into a porous irregular shaped iron oxide—commonly known as a “sinter”—that is feedstock for a blast furnace used in the production of pig iron, and eventually the production of steel.
Conventional sinter blend compositions have some disadvantages. For example, the energy source for conventional sintering processes is the carbon contained in the sinter blend composition's coke breeze (dust and fines of coke). However, as a result of the rapid growth of the steel industry worldwide, coke breeze is becoming increasingly expensive and difficult to procure. Another issue with sinter blend compositions is that after the sintering process they generally must produce a sinter with an ISO 3271 tumble strength rating (“ISO tumble strength”) typically, greater than 72, that is capable of withstanding the rigors of the blast furnace iron-making process, which involves movement of the sinter on conveyor belts, and into shaft furnaces, under significant weight compression. Accordingly, not all iron making reverts are easily repurposed by incorporation into a sinter blend composition, in evaluating a typical sinter blend it may be necessary to estimate the total energy content of the sinter blend composition. Depending on the blend, any of the following energy sources may be present: coke breeze, component metallics and component carbon. If the combined energy content of the sinter blend composition is too high, excessive temperatures and slagging may develop during the sintering process.
Direct reduced iron (DRI), sometimes called sponge iron, is a commercial product widely used as a source material for making steel. The conventional techniques for making steel involve the use of an electric arc furnace (EAF) or a basic oxygen furnace (BOF). DRI is typically higher in iron units than taconite pellets and other sources of iron, and can be used as a partial substitute for scrap in the production of steel by EAF.
DRI is formed from beneficiated iron ore, such as taconite pellets. For example, taconite has been mined and crushed, and the iron containing portions magnetically separated from the non-iron containing portions to form a beneficiated product higher in iron content than mined taconite. The beneficiated iron ore portion may be formed into pellets by pelletizing, and heated in a linear hearth furnace in the presence of reducing agent (e.g., carbonaceous material) to a temperature below the melting point of iron using natural gas or coal, to promote the reduction of iron ore to metallic iron. DRI is typically above 90% metallic iron with the remainder gangue.
In the process to make DRI, the beneficiated and pelletized iron oxide containing material is moved through a furnace mixed with a reducing agent, such as coal, coke, or another for of carbonaceous material. A desulfurizing agent, such as limestone or dolomite, is also typically added. The carbon of the reducing agent and the oxygen of the iron oxide material react chemically in the reducing zone of the furnace, thereby partially reducing the iron oxide to form metallic iron. This, and other traditional reducing processes, are used to create the DRI.
DRI is difficult to transport because DRI and DRI fines are highly reactive with oxygen in air and moisture. Moisture, in particular, reacts with the iron forming FeO and H2. The DRI being sponge iron his many voids making it porous in nature. The porous nature of DRI also means that it has low compressive strength, and handling of DRI generates surface fines and dust. Additionally, when the DRI is stored, for example in the hold of a ship during transportation, some of the pellets have been prone to disintegrate under the weight of pellets above them further generating fines and small particles. The DRI lines and small particles increased the ability for reaction with moisture and oxygen around it. Additionally, the rough surface characteristics of the DRI pellets produce particulate matter and other fines having a high surface area, which also promoted the likelihood of the DRI reacting with oxygen. Such particulate matter and fines typically are produced throughout the storage and transportation of the DRI, making it difficult to transport DRI over long distances and to store DRI for long periods.
The porous, low internal strength, and flakey nature of DRI all increase the surface area of the nodule that is exposed to an oxidizing atmosphere and/or moisture, resulting in substantial and rapid oxidation and rusting. The reactions that occur during DRI oxidation produce heat and hydrogen making DRI susceptible to overheating and combustion. Increases in temperature in containers storing DRI, in which air is free to circulate, can reach 1200° F. Such combustion causes fires in the holds of ships during transportation of DRI and in the clam buckets of cranes when unloading DRI. These risks have substantially increased the cost of DRI delivered to a steel plant because of the losses during transportation and storage. Due to the difficulties and risks associated with transporting DRI, production of DRI has with a few exceptions been generally located near the steelmaking facilities and near the time of use in steelmaking, rather than in more economical locations and times. For these same reasons, the disposal of DRI fines and dusts is expensive and difficult.
While disposing of DRI reverts is expensive and environmentally challenging, it has now been determined that DRI fines and dust (DF blend) can be successfully repurposed by being incorporated and used as a replacement energy source to replace coke breeze in sinter blends. Accordingly, in cokeless sinter blends disclosed herein, the DRI reverts are used as a replacement fuel source for the coke breeze in the sintering process, while still producing sinter with an ISO tumble strength of at least 72. The cokeless sinter blends are inert, safe to transport, inexpensive to produce and provide sinter with the ISO tumble strength ratings necessary for use in conventional blast furnace iron-making processes.
As used herein, “cokeless” or “free of coke breeze” means no intentional addition of coke or coke breeze.